Related
This might be a silly question.
When I am using Optional.isEmpty() like below in the code
Optional<List<String>> optional = Optional.of(new ArrayList<>());
optional.isEmpty(); // only checks if the value is NULL or not.
isEmpty() method is only checking whether the value null or not.
public boolean isEmpty() {
return value == null;
}
This method name seems not clear for me.
I'm wondering why this method is named isEmpty() and not isNull() since it performs null-check under the hood?
It’s the because the null check is an implementation detail, not what the method logically does. Logically, the method checks whether the optional value is empty or engaged.
Technically, this can be implemented in a number of ways, depending on how the Optional stores its value. JDK 8’s implementation happens to use a reference object with a null check, but other implementations could conceivably do this differently. It’s a mistake to confuse a high-level interface description with its implementation. The entire point of encapsulation is to abstract away implementation details from the user.
You're missing the main point.
Optional is designed to be a container of data which can never be null by itself and which is safe to interact with.
Optional is not intended to be a substitution for null-checks. It's mean to represent a nullable return value and should not be creating in order to hide null-checks or chain methods on it (more on that, see here and here).
If you need to validate that something is not null you can use method Objects#requireNonNull() and it's flavors or explicit null-check.
The idea behind the optional, is that serves as a box for a value, and you can do whole lot of staff with it without caring if the box empty or not. Optional offers you methods to filter to the value inside the box filter(), to replace an empty box with another box or(), to transform the value inside the box map(), etc. And all these actions can be done by chaining the methods fluently without opening the box.
Methods isNull()/isNotNull() are not suitable for the Optional API because they contradict its design goal, which to provide a limited mechanism for representing a nullable return value and a mean of interaction with the target value (around which optional is wrapped) in a simple and fluent way regardless whether its value is null or not.
Note, that checks isPresent() (JDK 8) and isEmpty() (JDK 11) exist as a last resort. Be attentive when you're using them, because most likely you're not leveraging Optional to its full power.
Also, note that wrapping Optional around a Collection or an array doesn't make sense because they are container of data as well (like optional) and might be empty as well, which represents the absents of data.
Doing something like this is redundant and inconvenient.
Optional<List<String>> optional = Optional.of(new ArrayList<>());
When a collection is not null, which should be the case because keeping nullable collections is an antipattern, Collection.isEmpty() is sufficient to check if the data is present. Meanwhile, when you have optional wrapped around collection, the presence of collection inside the optional doesn't means that the actual data is exists.
This is the definition of Optional:
A container object which may or may not contain a non-null value.
Since the object itself is a container, which may or may not hold a value, isEmpty (or isPresent) refers to the status of the container, whether it holds something or not.
isNull as a name would suggest that the Optional would be null, which would be a false impression, because the Optional itself is a properly existing instance.
Is the Optional null if it does not hold a not-null value? No, you can check for
myOptional == null
and you will see it's false (you couldn't even call isEmpty otherwise). Is the object inside the container null? If isEmpty is null, then it's a sure thing. Hence, the Optional is either empty, or something is present inside of it.
Here's a different way of implementing Optional which checks emptiness by using different concrete classes for full and empty instances.
public interface MyOptional<T> {
boolean isEmpty();
T getOrDefault(T aDefault);
static <T> MyOptional<T> empty() {
return new MyOptional<T>() {
#Override
public boolean isEmpty() {
return true;
}
#Override
public T getOrDefault(T aDefault) {
return aDefault;
}
};
}
static <T> MyOptional<T> of(T t) {
return new MyOptional<T>() {
#Override
public boolean isEmpty() {
return false;
}
#Override
public T getOrDefault(T aDefault) {
return t;
}
};
}
public static void main(String[] args) {
MyOptional<String> optString = MyOptional.empty();
System.out.println(optString.isEmpty());
}
}
This has a semantic difference from Java's Optional: this can contain a null value in a full instance, while Java's cannot.
Having been using Java 8 now for 6+ months or so, I'm pretty happy with the new API changes. One area I'm still not confident in is when to use Optional. I seem to swing between wanting to use it everywhere something may be null, and nowhere at all.
There seem to be a lot of situations when I could use it, and I'm never sure if it adds benefits (readability / null safety) or just causes additional overhead.
So, I have a few examples, and I'd be interested in the community's thoughts on whether Optional is beneficial.
1 - As a public method return type when the method could return null:
public Optional<Foo> findFoo(String id);
2 - As a method parameter when the param may be null:
public Foo doSomething(String id, Optional<Bar> barOptional);
3 - As an optional member of a bean:
public class Book {
private List<Pages> pages;
private Optional<Index> index;
}
4 - In Collections:
In general I don't think:
List<Optional<Foo>>
adds anything - especially since one can use filter() to remove null values etc, but are there any good uses for Optional in collections?
Any cases I've missed?
The main design goal of Optional is to provide a means for a function returning a value to indicate the absence of a return value. See this discussion. This allows the caller to continue a chain of fluent method calls.
This most closely matches use case #1 in the OP's question. Although, absence of a value is a more precise formulation than null since something like IntStream.findFirst could never return null.
For use case #2, passing an optional argument to a method, this could be made to work, but it's rather clumsy. Suppose you have a method that takes a string followed by an optional second string. Accepting an Optional as the second arg would result in code like this:
foo("bar", Optional.of("baz"));
foo("bar", Optional.empty());
Even accepting null is nicer:
foo("bar", "baz");
foo("bar", null);
Probably the best is to have an overloaded method that accepts a single string argument and provides a default for the second:
foo("bar", "baz");
foo("bar");
This does have limitations, but it's much nicer than either of the above.
Use cases #3 and #4, having an Optional in a class field or in a data structure, is considered a misuse of the API. First, it goes against the main design goal of Optional as stated at the top. Second, it doesn't add any value.
There are three ways to deal with the absence of a value in an Optional: to provide a substitute value, to call a function to provide a substitute value, or to throw an exception. If you're storing into a field, you'd do this at initialization or assignment time. If you're adding values into a list, as the OP mentioned, you have the additional choice of simply not adding the value, thereby "flattening" out absent values.
I'm sure somebody could come up with some contrived cases where they really want to store an Optional in a field or a collection, but in general, it is best to avoid doing this.
I'm late to the game but for what it's worth, I want to add my 2 Cents. They go against the design goal of Optional, which is well summarized by Stuart Marks's answer, but I'm still convinced of their validity (obviously).
Use Optional Everywhere
In General
I wrote an entire blog post about using Optional but it basically comes down to this:
design your classes to avoid optionality wherever feasibly possible
in all remaining cases, the default should be to use Optional instead of null
possibly make exceptions for:
local variables
return values and arguments to private methods
performance critical code blocks (no guesses, use a profiler)
The first two exceptions can reduce the perceived overhead of wrapping and unwrapping references in Optional. They are chosen such that a null can never legally pass a boundary from one instance into another.
Note that this will almost never allow Optionals in collections which is almost as bad as nulls. Just don't do it. ;)
Regarding your questions
Yes.
If overloading is no option, yes.
If other approaches (subclassing, decorating, ...) are no option, yes.
Please no!
Advantages
Doing this reduces the presence of nulls in your code base, although it does not eradicate them. But that is not even the main point. There are other important advantages:
Clarifies Intent
Using Optional clearly expresses that the variable is, well, optional. Any reader of your code or consumer of your API will be beaten over the head with the fact that there might be nothing there and that a check is necessary before accessing the value.
Removes Uncertainty
Without Optional the meaning of a null occurrence is unclear. It could be a legal representation of a state (see Map.get) or an implementation error like a missing or failed initialization.
This changes dramatically with the persistent use of Optional. Here, already the occurrence of null signifies the presence of a bug. (Because if the value were allowed to be missing, an Optional would have been used.) This makes debugging a null pointer exception much easier as the question of the meaning of this null is already answered.
More Null Checks
Now that nothing can be null anymore, this can be enforced everywhere. Whether with annotations, assertions or plain checks, you never have to think about whether this argument or that return type can be null. It can't!
Disadvantages
Of course, there is no silver bullet...
Performance
Wrapping values (especially primitives) into an extra instance can degrade performance. In tight loops this might become noticeable or even worse.
Note that the compiler might be able to circumvent the extra reference for short lived lifetimes of Optionals. In Java 10 value types might further reduce or remove the penalty.
Serialization
Optional is not serializable but a workaround is not overly complicated.
Invariance
Due to the invariance of generic types in Java, certain operations become cumbersome when the actual value type is pushed into a generic type argument. An example is given here (see "Parametric polymorphism").
Personally, I prefer to use IntelliJ's Code Inspection Tool to use #NotNull and #Nullable checks as these are largely compile time (can have some runtime checks) This has lower overhead in terms of code readability and runtime performance. It is not as rigorous as using Optional, however this lack of rigour should be backed by decent unit tests.
public #Nullable Foo findFoo(#NotNull String id);
public #NotNull Foo doSomething(#NotNull String id, #Nullable Bar barOptional);
public class Book {
private List<Pages> pages;
private #Nullable Index index;
}
List<#Nullable Foo> list = ..
This works with Java 5 and no need to wrap and unwrap values. (or create wrapper objects)
I think the Guava Optional and their wiki page puts it quite well:
Besides the increase in readability that comes from giving null a name, the biggest advantage of Optional is its idiot-proof-ness. It forces you to actively think about the absent case if you want your program to compile at all, since you have to actively unwrap the Optional and address that case. Null makes it disturbingly easy to simply forget things, and though FindBugs helps, we don't think it addresses the issue nearly as well.
This is especially relevant when you're returning values that may or may not be "present." You (and others) are far more likely to forget that other.method(a, b) could return a null value than you're likely to forget that a could be null when you're implementing other.method. Returning Optional makes it impossible for callers to forget that case, since they have to unwrap the object themselves for their code to compile.
-- (Source: Guava Wiki - Using and Avoiding null - What's the point?)
Optional adds some overhead, but I think its clear advantage is to make it explicit
that an object might be absent and it enforces that programmers handle the situation. It prevents that someone forgets the beloved != null check.
Taking the example of 2, I think it is far more explicit code to write:
if(soundcard.isPresent()){
System.out.println(soundcard.get());
}
than
if(soundcard != null){
System.out.println(soundcard);
}
For me, the Optional better captures the fact that there is no soundcard present.
My 2¢ about your points:
public Optional<Foo> findFoo(String id); - I am not sure about this. Maybe I would return a Result<Foo> which might be empty or contain a Foo. It is a similar concept, but not really an Optional.
public Foo doSomething(String id, Optional<Bar> barOptional); - I would prefer #Nullable and a findbugs check, as in Peter Lawrey's answer - see also this discussion.
Your book example - I am not sure if I would use the Optional internally, that might depend on the complexity. For the "API" of a book, I would use an Optional<Index> getIndex() to explicitly indicate that the book might not have an index.
I would not use it in collections, rather not allowing null values in collections
In general, I would try to minimize passing around nulls. (Once burnt...)
I think it is worth to find the appropriate abstractions and indicate to the fellow programmers what a certain return value actually represents.
From Oracle tutorial:
The purpose of Optional is not to replace every single null reference in your codebase but rather to help design better APIs in which—just by reading the signature of a method—users can tell whether to expect an optional value. In addition, Optional forces you to actively unwrap an Optional to deal with the absence of a value; as a result, you protect your code against unintended null pointer exceptions.
In java, just don't use them unless you are addicted to functional programming.
They have no place as method arguments (I promess someone one day will pass you a null optional, not just an optional that is empty).
They make sense for return values but they invite the client class to keep on stretching the behavior-building chain.
FP and chains have little place in an imperative language like java because it makes it very hard to debug, not just to read. When you step to the line, you can't know the state nor intent of the program; you have to step into to figure it out (into code that often isn't yours and many stack frames deep despite step filters) and you have to add lots of breakpoints down to make sure it can stop in the code/lambda you added, instead of simply walking the if/else/call trivial lines.
If you want functional programming, pick something else than java and hope you have the tools for debugging that.
1 - As a public method return type when the method could return null:
Here is a good article that shows usefulness of usecase #1. There this code
...
if (user != null) {
Address address = user.getAddress();
if (address != null) {
Country country = address.getCountry();
if (country != null) {
String isocode = country.getIsocode();
isocode = isocode.toUpperCase();
}
}
}
...
is transformed to this
String result = Optional.ofNullable(user)
.flatMap(User::getAddress)
.flatMap(Address::getCountry)
.map(Country::getIsocode)
.orElse("default");
by using Optional as a return value of respective getter methods.
Here is an interesting usage (I believe) for... Tests.
I intend to heavily test one of my projects and I therefore build assertions; only there are things I have to verify and others I don't.
I therefore build things to assert and use an assert to verify them, like this:
public final class NodeDescriptor<V>
{
private final Optional<String> label;
private final List<NodeDescriptor<V>> children;
private NodeDescriptor(final Builder<V> builder)
{
label = Optional.fromNullable(builder.label);
final ImmutableList.Builder<NodeDescriptor<V>> listBuilder
= ImmutableList.builder();
for (final Builder<V> element: builder.children)
listBuilder.add(element.build());
children = listBuilder.build();
}
public static <E> Builder<E> newBuilder()
{
return new Builder<E>();
}
public void verify(#Nonnull final Node<V> node)
{
final NodeAssert<V> nodeAssert = new NodeAssert<V>(node);
nodeAssert.hasLabel(label);
}
public static final class Builder<V>
{
private String label;
private final List<Builder<V>> children = Lists.newArrayList();
private Builder()
{
}
public Builder<V> withLabel(#Nonnull final String label)
{
this.label = Preconditions.checkNotNull(label);
return this;
}
public Builder<V> withChildNode(#Nonnull final Builder<V> child)
{
Preconditions.checkNotNull(child);
children.add(child);
return this;
}
public NodeDescriptor<V> build()
{
return new NodeDescriptor<V>(this);
}
}
}
In the NodeAssert class, I do this:
public final class NodeAssert<V>
extends AbstractAssert<NodeAssert<V>, Node<V>>
{
NodeAssert(final Node<V> actual)
{
super(Preconditions.checkNotNull(actual), NodeAssert.class);
}
private NodeAssert<V> hasLabel(final String label)
{
final String thisLabel = actual.getLabel();
assertThat(thisLabel).overridingErrorMessage(
"node's label is null! I didn't expect it to be"
).isNotNull();
assertThat(thisLabel).overridingErrorMessage(
"node's label is not what was expected!\n"
+ "Expected: '%s'\nActual : '%s'\n", label, thisLabel
).isEqualTo(label);
return this;
}
NodeAssert<V> hasLabel(#Nonnull final Optional<String> label)
{
return label.isPresent() ? hasLabel(label.get()) : this;
}
}
Which means the assert really only triggers if I want to check the label!
Optional class lets you avoid to use null and provide a better alternative:
This encourages the developer to make checks for presence in order to avoid uncaught NullPointerException's.
API becomes better documented because it's possible to see, where to expect the values which can be absent.
Optional provides convenient API for further work with the object:
isPresent(); get(); orElse(); orElseGet(); orElseThrow(); map(); filter(); flatmap().
In addition, many frameworks actively use this data type and return it from their API.
An Optional has similar semantics to an unmodifiable instance of the Iterator design pattern:
it might or might not refer to an object (as given by isPresent())
it can be dereferenced (using get()) if it does refer to an object
but it can not be advanced to the next position in the sequence (it has no next() method).
Therefore consider returning or passing an Optional in contexts where you might previously have considered using a Java Iterator.
Here are some of the methods that you can perform on an instance of Optional<T>:
map
flatMap
orElse
orElseThrow
ifPresentOrElse
get
Here are all the methods that you can perform on null:
(there are none)
This is really an apples to oranges comparison: Optional<T> is an actual instance of an object (unless it is null… but that would probably be a bug) while null is an aborted object. All you can do with null is check whether it is in fact null, or not. So if you like to use methods on objects, Optional<T> is for you; if you like to branch on special literals, null is for you.
null does not compose. You simply can’t compose a value which you can only branch on. But Optional<T> does compose.
You can, for instance, make arbitrary long chains of “apply this function if non-empty” by using map. Or you can effectively make an imperative block of code which consumes the optional if it is non-empty by using ifPresent. Or you can make an “if/else” by using ifPresentOrElse, which consumes the non-empty optional if it is non-empty or else executes some other code.
…And it is at this point that we run into the true limitations of the language in my opinion: for very imperative code you have to wrap them in lambdas and pass them to methods:
opt.ifPresentOrElse(
string -> { // if present...
// ...
}, () -> { // or else...
// ...
}
);
That might not be good enough for some people, style-wise.
It would be more seamless if Optional<T> was an algebraic data type that we could pattern match on (this is obviously pseudo-code:
match (opt) {
Present(str) => {
// ...
}
Empty =>{
// ...
}
}
But anyway, in summary: Optional<T> is a pretty robust empty-or-present object. null is just a sentinel value.
Subjectively disregarded reasons
There seems to be a few people who effectively argue that efficiency should determine whether one should use Optional<T> or branch on the null sentinel value. That seems a bit like making hard and fast rules on when to make objects rather than primitives in the general case. I think it’s a bit ridiculous to use that as the starting point for this discussion when you’re already working in a language where it’s idiomatic to make objects left-and-right, top to bottom, all the time (in my opinion).
I do not think that Optional is a general substitute for methods that potentially return null values.
The basic idea is: The absence of a value does not mean that it potentially is available in the future. It's a difference between findById(-1) and findById(67).
The main information of Optionals for the caller is that he may not count on the value given but it may be available at some time. Maybe it will disappear again and comes back later one more time. It's like an on/off switch. You have the "option" to switch the light on or off. But you have no option if you do not have a light to switch on.
So I find it too messy to introduce Optionals everywhere where previously null was potentially returned. I will still use null, but only in restricted areas like the root of a tree, lazy initialization and explicit find-methods.
Seems Optional is only useful if the type T in Optional is a primitive type like int, long, char, etc. For "real" classes, it does not make sense to me as you can use a null value anyway.
I think it was taken from here (or from another similar language concept).
Nullable<T>
In C# this Nullable<T> was introduced long ago to wrap value types.
1 - As a public method return type when the method could return null:
This is the intended use case for Optional, as seen in the JDK API docs:
Optional is primarily intended for use as a method return type where
there is a clear need to represent "no result," and where using null
is likely to cause errors.
Optional represents one of two states:
it has a value (isPresent returns true)
it doesn't have a value (isEmpty returns true)
So if you have a method that returns either something or nothing, this is the ideal use case for Optional.
Here's an example:
Optional<Guitarist> findByLastName(String lastName);
This method takes a parameter used to search for an entity in the database. It's possible that no such entity exists, so using an Optional return type is a good idea since it forces whoever is calling the method to consider the empty scenario. This reduces chances of a NullPointerException.
2 - As a method parameter when the param may be null:
Although technically possible, this is not the intended use case of Optional.
Let's consider your proposed method signature:
public Foo doSomething(String id, Optional<Bar> barOptional);
The main problem is that we could call doSomething where barOptional has one of 3 states:
an Optional with a value e.g. doSomething("123", Optional.of(new Bar())
an empty Optional e.g. doSomething("123", Optional.empty())
null e.g. doSomething("123", null)
These 3 states would need to be handled in the method implementation appropriately.
A better solution is to implement an overloaded method.
public Foo doSomething(String id);
public Foo doSomething(String id, Bar bar);
This makes it very clear to the consumer of the API which method to call, and null does not need to be passed.
3 - As an optional member of a bean:
Given your example Book class:
public class Book {
private List<Pages> pages;
private Optional<Index> index;
}
The Optional class variable suffers from the same issue as the Optional method parameter discussed above. It can have one of 3 states: present, empty, or null.
Other possible issues include:
serialization: if you implement Serializable and try to serialize an object of this class, you will encounter a java.io.NotSerializableException since Optional was not designed for this use case
transforming to JSON: when serializing to JSON an Optional field may get mapped in an undesirable way e.g. {"empty":false,"present":true}.
Although if you use the popular Jackson library, it does provide a solution to this problem.
Despite these issues, Oracle themselves published this blog post at the time of the Java 8 Optional release in 2014. It contains code examples using Optional for class variables.
public class Computer {
private Optional<Soundcard> soundcard;
public Optional<Soundcard> getSoundcard() { ... }
...
}
In the following years though, developers have found better alternatives such as implementing a getter method to create the Optional object.
public class Book {
private List<Pages> pages;
private Index index;
public Optional<Index> getIndex() {
return Optional.ofNullable(index);
}
}
Here we use the ofNullable method to return an Optional with a value if index is non-null, or otherwise an empty Optional.
4 - In Collections:
I agree that creating a List of Optional (e.g. List<Optional<Foo>>) doesn't add anything.
Instead, just don't include the item in the List if it's not present.
This is probably more a question about functional programming than about Java 8 specifically, but it's what I'm using right now.
I have a source object (could represent a repository, or a session..., doesn't matter here) that has a method retrieveSomething() that returns an Optional<SomethingA>.
I have a method somewhere that returns a Something, by calling retrieveSomething() and providing a default value in case the optional was empty, as follows:
return source.retrieveSomething()
.orElseGet(() -> provideDefaultValue());
Now I want to modify this code so that in case the source didn't contain any value yet (so the optional was empty), the source is updated with the provided default value.
Of course, I could easily do that inside a lambda expression code block:
return source.retrieveSomething()
.orElseGet(() -> {
Something sth = provideDefaultValue()
source.putSomething(sth);
return sth;
});
But if I understand correctly, I'm not supposed to use functions that cause side effects. So what's the "correct" (functional) way to do this, keeping the benefit of using Optional (in real code I'm actually also performing a map operation on it, but that's irrelevant here) ?
You could follow the way Java does this with Map.computeIfAbsent()
which takes a second parameter which is a function on how to compute and insert the record:
So your code would become:
Something something = source.computeIfAbsent(sth, (k)->provideDefaultValue());
An advantage of using a lambda to compute the default instead of just passing it in, is the lambda will only be evaluated if it needs to be so if computing the default is expensive, you only have to pay it when you need it.
From a conceptual standpoint, you use the optional pattern to deal with the absence of a return value. This means, if your source instance doesn't contain a value for you to use, you have the choice of providing a default value to use in its place.
It is not advised to modify the Optional directly to provide its value; that instance may be temporal and will differ on subsequent calls to retrieve it.
Since a function call truly governs what's returned by that Optional, the only approach you have if you truly want to go down this route is to modify how that value is computed. This really should be done from within the function providing the Optional, but it could be done outside of it if necessary.
Since there's not enough code structure here to truly write up some example, I will describe the steps:
Outside of the method providing the Optional, you write the same closure as you did before, with the side effect of adjusting the value used to compute the original Optional. This is likely a field of some sort.
Inside of the method providing the Optional, you ensure that you don't expose provideDefaultValue anywhere else (since they won't need it), and use a boolean conditional before you package the Optional.
return value == null ? Optional.of(provideDefaultValue()) : Optional.of(value);
...but that really defeats the purpose of the Optional, as you're still doing a null check.
A slightly better approach to the above would be to compute value in such a way that it was either itself or the default value, and return the Optional of that...
value = computeValue();
if(null == value) {
value = provideDefaultValue();
}
return Optional.of(value);
...but again, seriously defeating the purpose of Optional, as you're doing null checks.
An answer I came up with myself, which I'm not entirely satisfied with, but may serve as an example of what I'm looking for:
I could implement something similar to a Pair<V1,V2> class and then map the Optional<Something> to a Pair<Something, Boolean> where the Boolean value would indicate whether or not the value was a generated default:
Pair<Something, Boolean> somethingMaybeDefault =
source.retrieveSomething()
.map(sth -> new Pair<Something, Boolean>(sth, false))
.orElseGet(() -> new Pair<Something, Boolean>(provideDefaultValue(), true));
Then I'd update in case the boolean is false:
if (somethingMaybeDefault.value2()) {
source.putSomething(somethingMaybeDefault.value1());
}
And finally return the new value:
return somethingMaybeDefault.value1();
Of course, this uses imperative style for the update, but at least the functions remain pure.
I'm not sure this is the best possible answer though.
I've recently read about this and seen people using this class, but in pretty much all cases, using null would've worked as well - if not more intuitively. Can someone provide a concrete example where Optional would achieve something that null couldn't or in a much cleaner way? The only thing I can think of is to use it with Maps that don't accept null keys, but even that could be done with a side "mapping" of null's value. Can anyone provide me with a more convincing argument?
Guava team member here.
Probably the single biggest disadvantage of null is that it's not obvious what it should mean in any given context: it doesn't have an illustrative name. It's not always obvious that null means "no value for this parameter" -- heck, as a return value, sometimes it means "error", or even "success" (!!), or simply "the correct answer is nothing". Optional is frequently the concept you actually mean when you make a variable nullable, but not always. When it isn't, we recommend that you write your own class, similar to Optional but with a different naming scheme, to make clear what you actually mean.
But I would say the biggest advantage of Optional isn't in readability: the advantage is its idiot-proof-ness. It forces you to actively think about the absent case if you want your program to compile at all, since you have to actively unwrap the Optional and address that case. Null makes it disturbingly easy to simply forget things, and though FindBugs helps, I don't think it addresses the issue nearly as well. This is especially relevant when you're returning values that may or may not be "present." You (and others) are far more likely to forget that other.method(a, b) could return a null value than you're likely to forget that a could be null when you're implementing other.method. Returning Optional makes it impossible for callers to forget that case, since they have to unwrap the object themselves.
For these reasons, we recommend that you use Optional as a return type for your methods, but not necessarily in your method arguments.
(This is totally cribbed, by the way, from the discussion here.)
It really looks like the Maybe Monad pattern from Haskell.
You should read the following, Wikipedia Monad (functional programming):
And read From Optional to Monad with Guava on Kerflyn's Blog, which discusses about the Optional of Guava used as a Monad:
Edit:
With Java8, there's a built-in Optional that has monadic operators like flatMap. This has been a controversial subject but finally has been implemented.
See http://www.nurkiewicz.com/2013/08/optional-in-java-8-cheat-sheet.html
public Optional<String> tryFindSimilar(String s) //...
Optional<Optional<String>> bad = opt.map(this::tryFindSimilar);
Optional<String> similar = opt.flatMap(this::tryFindSimilar);
The flatMap operator is essential to allow monadic operations, and permits to easily chain calls that all return Optional results.
Think about it, if you used the map operator 5 times you would end up with an Optional<Optional<Optional<Optional<Optional<String>>>>>, while using flatMap would give you Optional<String>
Since Java8 I would rather not use Guava's Optional which is less powerful.
One good reason to use it is that it makes your nulls very meaningful. Instead of returning a null that could mean many things (like error, or failure, or empty,etc) you can put a 'name' to your null. Look at this example:
lets define a basic POJO:
class PersonDetails {
String person;
String comments;
public PersonDetails(String person, String comments) {
this.person = person;
this.comments = comments;
}
public String getPerson() {
return person;
}
public String getComments() {
return comments;
}
}
Now lets make use of this simple POJO:
public Optional<PersonDetails> getPersonDetailstWithOptional () {
PersonDetails details = null; /*details of the person are empty but to the caller this is meaningless,
lets make the return value more meaningful*/
if (details == null) {
//return an absent here, caller can check for absent to signify details are not present
return Optional.absent();
} else {
//else return the details wrapped in a guava 'optional'
return Optional.of(details);
}
}
Now lets avoid using null and do our checks with Optional so its meaningful
public void checkUsingOptional () {
Optional<PersonDetails> details = getPersonDetailstWithOptional();
/*below condition checks if persons details are present (notice we dont check if person details are null,
we use something more meaningful. Guava optional forces this with the implementation)*/
if (details.isPresent()) {
PersonDetails details = details.get();
// proceed with further processing
logger.info(details);
} else {
// do nothing
logger.info("object was null");
}
assertFalse(details.isPresent());
}
thus in the end its a way to make nulls meaningful, & less ambiguity.
The most important advantage of Optional is that it adds more details to the contract between the implementer and caller of a function. For this reason is both useful for parameters and return type.
If you make the convention to always have Optional for possible null objects you add more clarifications to cases like:
Optional<Integer> maxPrime(Optional<Integer> from, Optional<Integer> to)
The contract here clearly specifies that there is a chance that a result is not returned but also shows that it will work with from and to as absent.
Optional<Integer> maxPrime(Optional<Integer> from, Integer to)
The contract specifies that the from is optional so an absent value might have a special meaning like start from 2. I can expect that a null value for the to parameter will throw an exception.
So the good part of using Optional is that the contract became both descriptive (similar with #NotNull annotation) but also formal since you must write code .get() to cope with Optional.
Is it a bad practice to pass NULL argument to methods or in other words should we have method definitions which allow NULL argument as valid argument.
Suppose i want two method
1. to retrieve list of all of companies
2. to retrieve list of companies based upon filter.
We can either have two methods like as below
List<Company> getAllCompaniesList();
List<Company> getCompaniesList(Company companyFilter);
or we can have one single method
List<Company> getCompaniesList(Company companyFilter);
here in second case, if argument is NULL then method return list of all of companies.
Beside question of good practice practically i see one more issue with later approach which is explained below.
I am implementing Spring AOP, in which i want to have some checks on arguments like
1. Is argument NULL ?
2. is size of collection 0?
There are some scenarios where we can not have null argument at all like for method
void addBranches(int companyId, List<Branch>);
This check can be performed very well by using Spring AOP by defining method like following
#Before(argNames="args", value="execution(* *)")
void beforeCall(JoinPoint joinPoint ,Object[] args )
{
foreach(Object obj in args)
{
if(obj == NULL)
{
throw new Exception("Argument NULL");
}
}
}
But problem i am facing is since i have defined some of methods which should accept NULL argument for multiple functionality of one single method as mentioned above for method List getCompaniesList(Company companyFilter);
So i can not apply AOP uniformly for all of methods and neither some expression for methods name match will be useful here.
Please let me know if more information is required or problem is not descriptive enough.
Thanks for reading my problem and giving thought upon it.
It's fine, in cases when there are too many overloaded methods. So instead of having all combinations of parameters, you allow some of them to be null. But if you do so, document this explicitly with
#param foo foo description. Can be null
In your case I'd have the two methods, where the first one invokes the second with a null argument. It makes the API more usable.
There is no strict line where to stop overloading and where to start relying on nullable parameters. It's a matter of preference. But note that thus your method with the most params will allow some of them to be nullable, so document this as well.
Also note that a preferred way to cope with multiple constructor parameters is via a Builder. So instead of:
public Foo(String bar, String baz, int fooo, double barr, String asd);
where each of the arguments is optional, you can have:
Foo foo = new FooBuilder().setBar(bar).setFooo(fooo).build();
I use a very simple rule:
Never allow null as an argument or return value on a public method.
I make use of Optional and Preconditions or AOP to enforce that rule.
This decision already saved me tons of hours bugfixing after NPE's or strange behaviour.
It's common practice, but there are ways of making your code clearer - avoiding null checks in sometimes, or moving them elsewhere. Look up the null object pattern - it may well be exactly what you need: http://en.m.wikipedia.org/wiki/Null_Object_pattern?wasRedirected=true
The rule is: simple interface, complicated implementation.
Design decisions about your API should be made by considering how the client code is likely to use it. If you expect to see either
getAllCompaniesList(null);
or
if (companyFilter == null) {
getAllCompaniesList();
} else {
getAllCompaniesList(companyFilter);
}
then you're doing it wrong. If the likely use-case is that the client code will, at the time it is written, either have or not have a filter, you should supply two entry points; if that decision is likely not made until run-time, allow a null argument.
Another approach that may be workable may be to have a CompanyFilter interface with an companyIsIncluded(Company) method that accepts a Company and returns true or false to say whether any company should be included. Company could implement the interface so that companyIsIncluded method's behavior mirrored equals(), but one could easily have a singleton CompanyFilter.AllCompanies whose companyIsIncluded() method would always return true. Using that approach, there's no need to pass a null value--just pass a reference to the AllComapnies singleton.